Solid-state laser system

ABSTRACT

A method of operating a q-switch RE,XAB laser includes: providing a pump bias current to a pump source, the pump source directed to an RE:XAB gain medium, the RE:XAB gain medium within a resonator cavity, where X is selected from Ca, Lu, Yb, Nd, Sm, Eu, Gd, Ga, Tb, Dy, Ho, Er, and where RE is selected from Lu, Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Pr, Tm, Cr, Ho, with a bias current level below a lasing threshold of the RE:XAB gain medium; providing a pump pulse to the gain medium, the pump pulse of the lasing threshold of the RE:XAB gain medium, the pump pulse causing the RE:XAB gain medium to emit a laser pulse; and reducing the pump bias current to at least below the gain medium lasing threshold, the combination of the pump bias, the pump pulse, and the pump reduction having a current profile.

RELATED APPLICATIONS

This application is a continuation-in-part of pending patent applicationSer. No. 14/679,884, filed on Apr. 6, 2015.

TECHNICAL FIELD

The present disclosure relates in general to solid state laser systems.The disclosure relates in particular to solid state laser systems.

BACKGROUND INFORMATION

Lasers are used in a variety of applications. Lasers are used inmedical, manufacturing, military, and consumer applications. One lasertype, solid-state lasers, are based on crystalline or glass rods dopedwith ions. The doped rods are within in a resonator cavity and pumped toexcited states which decay emitting laser light.

SUMMARY OF THE DISCLOSURE

The present disclosure is directed to a laser system and methods ofoperating a laser system. In one embodiment, a method of operating aq-switch RE,XAB laser includes: providing a pump bias current to a pumpsource, the pump source directed to an RE:XAB gain medium, the RE:XABgain medium within a resonator cavity, where X is selected from Ca, Lu,Yb, Nd, Sm, Eu, Gd, Ga, Tb, Dy, Ho, Er, and where RE is selected fromLu, Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Pr, Tm, Cr, Ho, including thelanthanide ions, with a bias current level below a lasing threshold ofthe RE:XAB gain medium; providing a pump pulse to the gain medium, thepump pulse of the lasing threshold of the RE:XAB gain medium, the pumppulse causing the RE:XAB gain medium to emit a laser pulse; and reducingthe pump bias current to at least below the gain medium lasingthreshold, the combination of the pump bias, the pump pulse, and thepump reduction having a current profile.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate preferred methods and embodimentsof the present disclosure. The drawings together with the generaldescription given above and the detailed description of preferredmethods and embodiments given below, serve to explain principles of thepresent disclosure.

FIG. 1 is a block diagram of a laser system with a RE:XAB gain medium,the laser system within a resonator cavity, a pumping source, thepumping source directed towards the gain medium, a heat spreader, theheat spreader in thermal communication with the gain medium, and a lasercontroller, the laser controller operating the laser system.

FIG. 2A is a plan view of an embodiment of the laser system with anend-pumped design.

FIG. 2B is a plan view of an embodiment of the laser system wherein thepump source is a laser diode integrated on a baseplate.

FIG. 2C is a plan view of an embodiment of the laser system wherein theheat spreader is not within the clear aperture of the RE:XAB gainmedium.

FIG. 2D is a plan view of the laser system wherein an off-axis activeoptical q-switching is employed.

FIG. 2E is a plan view of the laser system wherein on-axis activeoptical q-switching is employed.

FIG. 2F is a plan view of a double end-pumped laser system.

FIG. 3A is a graph of the absorption coefficient spectra of the RE:XABwherein RE is Er,Yb and X is Y for the π-polarization andσ-polarization.

FIG. 3B is a graph of the emission spectra of the π-polarization stateof the Er,Yb:YAB gain medium.

FIG. 3C is a graph of the emission spectra of the σ-polarization of theEr,Yb:YAB gain medium.

FIG. 4A is an exploded perspective view of one embodiment of the heatspreader wherein the RE:XAB crystal is in thermal communication with anoptically transparent window and a thermally conductive mount.

FIG. 4B is an exploded perspective view of another embodiment of theheat spreader wherein the RE:XAB similar to that shown in FIG. 4A,further comprising another thermally conductive mount in thermalcommunication with the gain medium.

FIG. 4C is an exploded perspective view of another embodiment of theheat spreader wherein the gain medium is sandwiched between twooptically transparent windows and thermally conductive mounts.

FIG. 4D is an exploded perspective view of one yet another embodiment ofthe heat spreader, similar to that shown in FIG. 4C, wherein anadditional thermally conductive mount is between the two opticallytransparent windows.

FIG. 4E is an exploded perspective view of the heat spreader wherein theheat spreader is not in contact with the clear aperture of the gainmedium.

FIG. 4F is cross-section view of another of the heat spreader, whereinthe heat spreader is not in contact with the clear aperture of the gainmedium.

FIG. 5A is a graph of a bias pump method operating the laser system.

FIG. 5B is a block diagram of the bias pump method steps illustrated inFIG. 5A

DETAILED DESCRIPTION OF EMBODIMENTS

Referring now to the drawings, wherein like components are designated bylike reference numerals. Methods and various embodiments of the presentinvention are described further hereinbelow.

FIG. 1 is a block diagram illustrating a preferred embodiment 10 of alaser system. Laser system 10 includes a doped RE:XAB gain medium 12,gain medium 12 within a resonator cavity 14. Gain medium 12 is inthermal communication with a heat spreader 18. A pumping source 16directs output towards gain medium 12. A laser controller 20 operatesthe laser system, by controlling a pump driver 21, the pump driverdelivering current to pump source 16.

The RE:XAB gain medium is a borate crystal wherein (X) is selected fromCa, Lu, Yb, Nd, Sm, Eu, Gd, Ga, Tb, Dy, Ho, or Er, (A) is aluminum, and(B) is borate with doped with (RE) rare earth elements. (RE) can beselected from Lu, Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Pr, Tm, Cr, Ho,including the lanthanide ions, or combinations thereof.

The gain medium is expressed through the present disclosure as RE:XAB.In some embodiments the XAB crystal is codoped and RE is understood tomean a plurality of dopants. The RE:XAB is manufactured using hightemperature dipping seed solution. The percentage of RE dopants dependson the concentrations of the RE dopants in the high temperature dippingseeded solution growth (DSSG) of the crystal using the flux method. Ingrowth, RE ions replace sites within the XAB crystal. One nonlimitingexample of a suitable dopant concentration for the present embodiment isEr at about 3 at. %. Er doping concentrations can range from about 1% toabout 25%. Doping ions, such as chromium (Cr), Iron (Fe) and gallium(Ga) can be incorporated within the XAB crystal during growth, replacingAl sites.

The gain medium crystal can be cut on both an a-cut or a c-cutcrystallographic axes. The c-cut RE:XAB gain medium has a smallerabsorption cross-section and absorbs more efficiently than the a-cutRE:XAB gain medium. When cut and polished the Re:XAB gain medium can beshaped with a cross-section that is curved edges, with straight edges,such as a polygon, or combinations thereof. Further the RE:XAB gainmedium cross-section can change to any desired shape along its lengthwith abrupt or smooth transitions. In one embodiment the RE:XAB gainmedium cross-section is circular with diameter about the size of theexpected pumping source illumination area.

RE:XAB gain medium 12 has absorption bands that are dependent on thedopants. In some embodiment absorption bands are in the near-infrared(NIR) and stimulated emission bands at longer wavelengths bands in thenear-infrared. The gain medium can be resonantly pumped at wavelengthwithin about 200 nanometers of the laser system emission wavelength.Through the present disclosure the wavelengths used to excite the gainmedium are generally referred to as pumping radiation. The wavelengthsemitted from the gain medium after excited state radiative decay aregenerally referred to as NIR long wavelength bands and called lasingemission, stimulated emission or laser pulse when the radiative decay iscaused by stimulated emission.

Nonlimiting design factors for the RE:XAB gain medium include thecrystal shape, diameter, thickness, RE doping concentration, RE dopingconcentration, Al replacement site doping, crystallographic axis cut,angular orientation with respect to pump polarization, surface coatingand surface shape.

In some embodiments the RE:XAB is codoped and RE is a plurality ofdopants. For instance, RE codopants can be Yb and Er. When opticallypumped the Yb ions absorb NIR short wavelength they efficiently transferthe absorbed energy to an excited state of the Er ions. Rapidnon-radiative decay of the Yb excited state populates the upper level ofthe Er laser transition. The Er laser transition can decay to its groundstate by radiative decay by stimulated emission.

Pumping source 16 outputs optical energy directed towards gain medium12. Pumping source 16 can be a lamp, led, laser diode, laser system, orother optical radiation source. Depending on the pumping source type,the optical output of pump source 16 can vary with respect towavelength, optical bandwidth, output angle, spatial intensity profile,angular intensity profile, polarization, or pump profile andcombinations thereof. Further a plurality of the pump sources can beemployed. The pumping source wavelength and optical bandwidth can bealtered based on the pumping source type and can be modified by, forinstance, dielectric coating, Bragg grating, or other such opticalfilter. Pump source 16 can be oriented to provide polarized, randomlypolarized or partially polarized optical radiation. When the light ispolarized or partially polarized, the polarization azimuth can be fixedat any angle or otherwise elliptically rotate.

Laser controller 20 operates and controls a pump driver 22, the pumpdriver delivers current to pump source 16. Pump source 16 can be drivenwith constant or modulated current profiles. A plurality of pump driverscan be employed for each of the pumping sources, when more than onepumping source is provided the current profile can be digital, analog,or combinations thereof. Specific modulation methods are discussed ingreater detail further hereinbelow.

The pump sources can be polarized, with multiple sources havingdifferent polarization. Sources can be delivered using fiber optics.

A pump delivery optic 24 receives optical output from pump source 16 anddirects the pumping radiation towards Re:XAB gain medium 12. The pumpdirects light emitted by the pump source 16 such that gain medium 12 isilluminated in an area about corresponding with the desired stimulatedemission beam size. Pump delivery optics 24 can include refractive,reflective, diffractive, waveguides, spatial filters, optical filters,and combinations thereof.

In addition to directing the pumping radiation to the gain medium, thepump delivery optics can include spatial filtering, fiber optic, andhomogenization optics in order to combine sources or to provide thedesired spatial illumination profile on the gain medium. Nonlimitingfactors in designing the pumping optics include the number of thepumping sources, desired wavelength, optical bandwidth, entrance angle,spatial intensity profile, angular intensity profile, polarization, andcombinations thereof. The pump radiation directed by pump deliveryoptics 24 enter resonator 14, and are directed to the RE:XAB gain mediumwithin the resonator.

Resonator 14 is defined by a first resonator surface 26 on an inputcoupler and a second resonator surface 28 on an output coupler. Theinput coupler transmits the pump radiation, allowing the pump radiationto enter gain medium 12. The input coupler at least partially reflectslaser radiation. Output coupler 28 at least partially reflects lasingradiation. In general, the output coupler is the surface that allowslaser pulses to exit the resonator, although in some embodiment theinput coupler allows pulses to exit and is therefore both the inputcoupler and the output coupler. In practice, the input and outputcoupler surfaces can be defined on any surfaces of the optical-elementswithin the laser system, including the gain medium, so long as multiplestimulated emission resonates through the gain medium. Resonator 14 canincorporate end-pump, side-pump, ring-oscillator and other suchresonator designs. Further a plurality of the input coupler and theoutput coupler surfaces can be employed.

Within the resonator is a q-switch 30. Q-switch 30 can be an active orpassive device. Passive q-switching techniques include saturableabsorber and thin-film absorbers including bulk nonlinear saturableabsorbers, semiconductor saturable absorber mirrors (SESAMs), saturableabsorber output coupler and related devices. Active q-switching includeelectro-optic, acousto-optic, mechanically rotating or switching,microelectromechanical systems, and hybrid optical driven passiveq-switching. When the q-switch is active, the laser controller operatesthe q-switch directly or through a driver. Nonlimiting factors indesigning the q-switch, although such considerations are coupled to thelaser system design as a whole, include the required numerical aperture,maximum pulse frequency modulation, pulse duration, pulse width,recovery time, peak pulse power, modulation depth, damage threshold, andabsorption coefficient.

RE:XAB Gain medium 12 is in thermal communication with a heat spreader18. Heat spreader 18 allows heat dissipation from RE:XAB gain medium 12when the laser system is operated at higher powers or otherwise allowtemperature control of the RE:XAB gain medium. Heat spreader 18 can bepart of a mechanical mount in thermal contact with the gain medium onthe crystal surfaces, perimeter, or both. The heat spreader can be athermally conductive epoxy, solder, or other curable substance that canact as both an adhesive and allow thermal transport. Alternatively theheat spreader can be an optically transparent window, at one of theoptically transparent window surfaces in contact with one of the RE:XABgain medium surfaces. The optically transparent window may be polarized.The optically transparent window is preferable refractive index matchedto the RE:XAB gain medium. The optically transparent window is chosenfrom materials with good thermal conduction characteristics, goodhardness, and good ability to withstand thermal shock. Such materialsinclude sapphire, diamond, synthetic diamond, or ceramic.

Thermal communication between the heat spreader and the RE:XAB gainmedium can be increased with thermal conductive epoxy, paste, solder,wettable metals such as indium, and techniques such as plating,conformal contact, optical contact, CVD deposition, or diffusion bondingwhen appropriate. Conformal contact refers to approximate surfacematching and can include adhesives. Optical contact refers to conformalsurface shaping wherein each surface is optically quality, typicallysurface conformance better than about 1-2 nanometers, and contaminantfree, such that intermolecular forces hold the surfaces together.Diffusion bonding is similar to optical contact except in addition tooptical contact, heating and pressure processes reaching up to 60%-80%of the melting temperature to allow atomic diffusion of elements betweenthe parts thereby forming a bond.

The heat spreader can be in thermal communication with a heat sink. Theheat sink or the heat spreader can be thermally controlled with passivecooling, active cooling, or combinations thereof. Passive cooling caninclude fins, rods, metal foam and other such structured surfaces. Suchfeatures can be placed on or within the heat spreader or a conductivemount, baseplate, or any heatsink which the heat spreader is also inthermal communication. Active cooling can include fans, circulatingfluids or thermoelectric coolers.

Optionally, a temperature monitor 32 can be placed on gain medium 12,heat spreader 18, pumping source 16, on any other element within thelaser system or in thermal communication with an element within lasersystem and combination thereof. Temperature monitor 32 can be athermistor and provide temperature feedback to laser controller 20.Laser controller 20 can then control operation of any active coolingdevices provided or otherwise alter operation of the laser system basedon previous characterization or concurrently measured laser performance.For instance, concurrent measured performance can be done by integratingan optical detector 34.

Optical detector 34 can measure optical radiation from either the pumpsource, scattered laser radiation, or direct laser radiation andcombinations thereof. Optical detector 34 can be a PIN photodiode,avalanche photodiode, piezo based detector, or any other detectorcapable of fast energy pulses detection. One semiconductor materialcapable of detecting NIR is InGaAs. Either a single or plurality ofoptical detectors can be implemented for detecting optical radiationfrom the laser system. Detected laser radiation can provide feedback tolaser controller 20. Laser controller 20 can then control operation ofthe pump driver, or any other controllable device provided, based on theoptical feedback.

In addition to the optical components specifically referenced, a varietyof optical components can be implemented within the laser system, placedat any position between the pumping source and the laser output. By wayof example, an optical component 36 can be placed between RE:XAB gainmedium 12 and Q-switch 30 or an optical component 38 can be placedbetween q-switch 30 and output coupler 28. The optional opticalcomponents can be a single or plurality of refractive lenses, reflectivemirrors, diffractive surfaces, spatial filters, spectral filters,polarizers, attenuators and combinations thereof. Introduction of suchelements allows modification of the laser emission, nonlimiting examplesof which include resonator characteristics, beam shape, beam size,allowed modes, mode mixing, divergence, wavelength, spectral bandwidth,and polarization.

Referring to FIG. 2A, one embodiment of the present invention, a lasersystem 50A has RE:XAB gain medium 12. RE:XAB gain medium 12 has a firstsurface 12F and a second surface 12S. Gain medium 12 resides withinresonator cavity 14. Resonator cavity 14 is defined by a first surface14F and a second 14S, the resonator cavity surfaces also being surfacesof components along an optical axis 54.

Here, the pumping source is a fiber coupled laser diode 54. Fibercoupled laser diode (LD) 54 has output directed towards RE:XAB gainmedium 12. In this embodiment, laser system 50A has an end-pump passiveq-switch design with fiber coupled LD 54 emitting optical radiationλ_(s) along optical-axis 52 towards gain medium 12. Optical radiation λsis in the absorption band of RE:XAB gain medium 12. Fiber coupled laserdiode 54 has an optical fiber 56 terminating in a cylindrical ferrule58. Ferrule 58 is mechanically fastened in 3-point contact mount 59,3-point mount 59 integrated within, or mechanically fastened to, abaseplate 61. Optical fiber 56 terminates at the laser diode package,the laser controller operating the fiber coupled laser diode via adriver, the laser controller and the pump driver not shown in thepresent view.

Fiber coupled LD source 54 can comprise of one emission source or aplurality of emission sources. For instance, the pump source can be asingle LED or semiconductor laser diode element, or a plurality of LEDor semiconductor laser diode elements. The laser diode sources can becoupled into a single optical fiber or a plurality of optical fibers allof which are directed towards the RE:XAB gain medium. The optical fiberscan be singlemode, polarization maintaining, or multimode. The opticalfiber can include features such as mode mixing structures, bragggratings, endcapping, coating, angled surface termination other suchfeatures.

Short wavelength optical radiation λ_(s), represented as short dashedmarginal rays, exit ferrule 58 according to the numerical aperture ofoptical fiber 56, or by the numerical aperture as modified by optionalend-capping termination, towards the pump delivery optics. Here the pumpdelivery optics is an aspheric optic 60. Aspheric optic 60 has a firstsurface 60F and a second surface 60S. Optical radiation λ_(s) refractsat aspheric optic surface 60F, propagates through the aspheric lens andrefracts at second surface 60S, converging towards heat spreader 18 andgain medium 12.

Here, the heat spreader is an optically transparent window 62. Opticallytransparent window 62 has a first surface 62F and a second surface 62S.Here, heat spreader second surface 18S is in physical contact with gainmedium first surface 12F, thereby in thermal communication. Opticalradiation λ_(s) enters heat spreader 62 refracting at window firstsurface 62F. Optically transparent window 62 is a transparent windowwith low absorption of optical radiation λ_(s). For sufficient thermalcontact with the RE:XAB gain medium, the optically transparent windowmay be bonded in conformal contact either with adhesives or bynon-adhesive contact techniques such as optical contact or diffusionbonded. The optically transparent window is connected to baseplate 61,the baseplate acting as a heat sink for the heat spreader and a mountingstructure for other optical components. In this configuration heatspreader 62 is within resonator 14 wherein window first surface 62F andfirst resonator surface 14F are the same surface. Window first surface62F is coated with a short wavelength anti-reflective (AR-coating)coating and a long wavelength high-reflectance coating (HR-coating),thereby allowing the pumping emission into the gain medium whileconfining stimulated emission from the gain medium within the resonator.When the heat spreader is in optical or diffusion contact with the gainmedium and the heat spreader is made from gain medium index-matchingmaterial, for instance undoped XAB, the short wavelength opticalradiation enters the gain medium with substantially no loss fromreflectance. Various embodiments and configurations of the heat spreaderare discussed in detail further hereinbelow

The short wavelength optical radiation enters the RE:XAB gain medium andthe gain medium absorbs the short wavelength radiation. The absorptionof the short wavelength optical radiation is dependent on thewavelength, pump polarization, crystallographic cut, stoichiometry, andorientation, the length and the illuminated cross-section of the RE:XABgain medium. RE:XAB gain medium second surface 12S can optionally have ashort wavelength HR-coating to reflect any unabsorbed short wavelengthradiation.

The absorbed short wavelength radiation populates the upper energystates of the RE:XAB gain medium. Initially a broad spectrum of longwavelength radiation is emitted. Some of the long wavelength emissionpropagates to the q-switch, here the q-switch is a saturable absorber64. Saturable absorber 64 has first surface 64F and second surface 64S.Saturable absorber 64 is a material that absorbs optical radiation up toa saturation point. Upon saturation the saturable absorber loses itsability to absorb, becoming suddenly transmissive. One suitablesaturable absorber for NIR is Cobalt Spinel (Co: Spinel). Cobalt spinelabsorbs from about 1100 nm to about 1600 nm. Alternatively, a saturableBragg absorber or reflector could be used.

As the gain medium upper energy levels populate and emit a longerwavelength radiation λ_(L), the saturable absorber saturates suddenlybecoming a low loss medium and transmits long wavelength radiationλ_(L). The long wavelength emission propagates to a output coupler 66.Output coupler 66 has a first surface 66F and a second surface 66S.Here, absorber first surface 66F is curved such that reflected radiationpropagates back through the resonator such that the reflected longwavelength radiation beam size is about the same size as the shortwavelength beam size illuminating the RE:XAB gain medium. First surface66F has a partially reflective coating wherein the partially reflectivecoating reflects a discrete band of long wavelength radiation. Thediscrete wavelength band reflects back to RE:XAB gain medium 12 causingstimulated emission of populated energy states in the RE:XAB gainmedium.

Lasing occurs when the RE:XAB medium is pumped sufficiently to causepopulation inversion, and losses become suddenly low due to saturationof the saturable absorber, and the roundtrip gain caused by simulatedemission within the resonator exceed losses. The energy stored in thepopulation inversion is emitted in a fast, intense laser pulse. Thelaser pulse transmits through output coupler second surface 36S. Afterpulse emission, and partial depletion of the RE:XAB energy states, thesaturable absorber recovers to high-loss state and another pulse iseventually emitted, depending on continued or modulated pump currentprovided to the pumping source.

Here, the optional optical monitor provided is an avalanche photodiode(APD) 72. APD 72 detects a scattered light 74 from the intense laserpulse. APD 72 is placed at the periphery of the laser system such thatthe APD is not directly in the optical path of the laser system. Placingthe APD at the periphery allows for detection of light scattered fromoptical elements during pulse emission. Using an APD allows detection ofoptical radiation at low levels. If scattered light is sufficient a PINor other optical detector can be used. APD 72 may connect to the lasercontrol and provide temporal feedback of the laser pulses. The opticalmonitor can provide temporal feedback of the pulse emission to the lasercontroller, which in turn can alter operation of the laser system basedon the feedback.

FIG. 2B illustrates a laser system 50B. Laser system 50B is similar tothat shown in FIG. 2A, except here the pumping source is an integratedlaser diode 80. Laser diode 80 is directly mounted on baseplate 61.Pumping radiation λ_(S) is emitted and coupled into the beam deliveryoptics. Here the beam delivery optics include an optional mode filteringoptic 86 and aspheric lens 60. Mode filtering optic 86 may includeoptical fiber, anamorphic optic, or other such beam shaping optic toallow modification of transverse modes. Such mode filtering optics allowfiltering of single-mode and multimode output pumping radiation in orderto achieve the desired beam profile. For instance, if the pumpingradiation emits an anamorphic beam, whether multimodal or single mode,the mode filtering optic can homogenize or strip the modes to provide asingle or multimodal beam structure in order to provide even circularillumination onto gain medium 12. If the mode filtering optic includes amultimodal optical fiber, mode scrambling can be implemented by pinchingoptical fiber in various locations in order to provide a high ordertransverse mode homogenized illumination.

Here a laser controller 82 is shown integrated on baseplate 61. Thelaser controller providing pump current to laser diode 80. Lasercontroller 82 is also in electrical connection with photodetector 72,temporally monitoring lasers pulses.

FIG. 2C is a plan view of a laser system 50C, another embodiment of thepresent disclosure. Laser system 50C is similar to that shown in FIG.2A, except here the heat spreader is a mechanical mount 90, themechanical mount in thermal communication with the side and optionallythe perimeter of gain medium 12, the mechanical mount not in contactwith the clear aperture. The clear aperture being the portion of thegain medium surface through which pumping or lasing radiation isintended to propagate. The clear aperture can be centered, off-center,symmetric and asymmetric with respect to the RE:XAB gain medium. Forinstance the pump radiation can be directed to an outer edge of theRE:XAB gain medium, the outer-edge being closer to bulk thermal spreadermaterial to allow faster thermal transport.

In laser system 50C, first resonator surface 14F is RE:XAB gain mediumfirst surface 12F, first surface 12F having an AR coating for the shortwavelength radiation and an HR coating for the long wavelengthradiation. Alternatively, the heat spreader can be a thermallyconductive epoxy, solder-based, or combinations thereof. The heatspreader can be thermally conductive epoxy, the thermally conductiveepoxy acting as both the heat spreader and mounting adhesive bonding thegain medium to the baseplate. The gain medium can be metalized onsurfaces outside the clear aperture and any edges, then solder mountedonto baseplate 61.

FIG. 2D is a plan view of a laser system 50D, similar to that shown inFIG. 2A, except here saturable absorber 64 is additionally controlledwith another radiation source. A saturable absorber optical driver 100has an optical radiation XSA directed towards saturable absorber 64.Saturable absorber optical driver 100 can be an LED, LD, laser or othersuch radiation source. The saturable absorber optical driver wavelengthmust be within the spectral absorption region of the saturable absorber64.

The laser controller operates saturable absorber optical driver 100,allowing increased control of when saturable absorber 64 becomestransmissive. Optical radiation λ_(SA) from the saturable absorberoptical driver can be held constant or modulated. In constant currentmode, intensity of optical output changes the frequency and pulse energyoutput from the laser system. Increased optical driver output radiationincreases pulse emission frequency and lowers pulse energy. Modulatingthe saturable absorber optical driver, in coordination with driving thepump source allows increased deterministic control over saturation ofthe saturable absorber. Such a hybrid passive/active q-switching allowscontrol over the pulse energy and temporal pulse emission.

FIG. 2E is a plan view of a laser system 50E, similar to that shown inFIG. 2D, except here a spectral beam combiner 102 directs from saturableabsorber optical driver 100 to saturable absorber 64. Spectral beamcombiner 102 is between RE:XAB gain medium 12 and saturable absorber 64.Here the spectral beam combiner is a cube and has a spectral filter onthe 45-degree interface 104, the spectral filter transmitting long NIRwavelength emission λ_(L) and reflecting saturable absorber driveremission λ_(SA). Having an on-axis spectral beam combiner allows bettermatching of the cross-section of the saturable absorber that isilluminated.

FIG. 2F is a plan view of a double end-pumped laser system 50F, similarto that shown in FIG. 2C, except here the pumping source comprisesanother fiber coupled laser diode 55. Fiber coupled laser diode 55 hasoutput directed towards RE:XAB gain medium 12 thereby providing a doubleend-pump configuration. In this embodiment, an coupler optic 105 is at a45-degree angle with respect to the resonator cavity and directs opticalradiation from pump source 55 to the RE:XAB gain medium and transmitslaser emission.

FIG. 3A is a graph 110 showing the polarized absorption coefficientspectra of the RE:XAB laser system, wherein RE is Er,YB and X is Y fromabout 900 nm to about 1025 nm. A a-polarized absorption coefficientspectra 112 has a relatively flat response with a substantially flatresponse region 114 at about 940 nm. A σ-polarization absorptioncoefficient spectra 116 has absorption with spectral absorption peak 118at about 976 nm with a full width half max (FWHM) 120 of about 17 nm. Asaforementioned, the amount of optical pumping output absorbed depends onthe pumping wavelength, pumping polarization, and thickness of the gainmedium. When pumped with σ-polarization at about 976 nm the thicknessfor the c-cut Er,Yb:YAB gain medium can be from about 100 to about 200microns (μm) due to the large absorption coefficient of about 184 cm⁻¹.To ensure pumping at about 976 nm with a laser diode based pumpingmechanism, the pump can be wavelength stabilized by either integrating adistributed Bragg reflector or by controlling the temperature of thedevice. When pumped with the π-polarized state, the thickness of theEr,Yb:YAB crystal has to be thicker relative to the σ-polarizationthickness to achieve similar absorption, but the pumping wavelength andvariations thereof during operation have less effect, especially whenpumped in flat response region 114 at about 940 nm.

FIG. 3B and FIG. 3C show the Er,Yb:YAB gain medium emission spectra fromabout 1450 nm to about 1500 nm for a specific average pump power. FIG.3A is graph 110B showing π-polarization emission spectra 122. Emissionspectra 122 has multiple emission peaks. FIG. 3C is a graph 110C showingσ-polarization emission spectra 124. Emission spectra 124 has multipleemission peaks with a strong peak 126 at about 1530 nm. Asaforementioned partially reflecting coating design on the first surfaceof the output coupler can be tuned to provide spectral emission at anyof the emission wavelengths, the efficiency of the laser increasing whenthe partially reflecting coating is designed for emission peaks, forexample 1530 nm, and the efficiency also increasing when the Er,Yb:YABcrystal orientation is oriented to preferred polarization angles.

The Er,Yb:YAB gain medium absorbs radiation in substantially similarbands as shown in FIG. 3A and FIG. 3B and the pumping source can providepumping radiation at those wavelengths. For example, the pumping sourcecan be a 1470 nm laser diode, with the output coupler having a partiallyreflective coating at a wavelength longer than 1470 nm. Implementingsuch a pumping source with pumping radiation in about proximity to thelasing emission wavelength reduces the amount of heat produced relativeto pumping at shorter wavelengths.

The heat spreader allows heat transfer from the gain medium when thelaser system is operated at high power. When excited states releaseenergy in nonradiative decay the energy is released as phonons. Whenoperating at high powers, for instance above about 1 watt (W) averagepower with 100% duty cycle, heat buildup on the gain medium can causethermal lensing effects and possibly cause damage to the crystal.Thermal lensing effects can lead to an instable resonator, limitingperformance and power. In general the heat spreader and any heat sink inthermal communication with the RE:XAB must be able to handle the heatgenerated in the crystal by the pump source. Heat generation depends onthe pump wavelength and the laser emission wavelength. Pump power toheat conversion can be from about 1% to about 45% of pump power. Theheat spreader embodiments described below allow high average poweroperation.

FIG. 4A shows embodiment of a heat spreader 150. RE:XAB gain medium 12is attached to a optically transparent window 152. Optically transparentwindow 152 has a first surface 152F and a second surface 152S. Opticallytransparent window 152 can be made from undoped XAB crystal or otheroptically transparent materials for either the pump source or emissionsource. Preferred material for the optically transparent window based onrefractive index matching, coefficient of thermal expansion (CTE)matching, and thermal conductivity. The first surface of gain medium 12is in thermal communication with optically transparent window 152 ineither conformal, optical contact, or diffusion bonded.

In this example, optical window 152 has a diameter larger than thediameter of gain medium 12. The surface area exposed allows thermalcontact to conductive mount 156. Conductive mount 156 has first surface156F and second surface 156S with thru-hole 158. The diameter ofthru-hole 158 is at least as large as the diameter of gain medium 12.Optically transparent window 152S is attached to conductive mount firstsurface 156F such that they are in thermal communication. Conductivemount 156 can be made from thermally conductive material such as metals,ceramics, and composite materials. Nonlimiting examples of such materialinclude copper, aluminum, aluminum nitride, and graphite/graphene/metalembedded polymers.

The method of attaching or fastening the optically transparent window tothe conductive mount can be mechanical or adhesive based, depending onthe materials used and expected operation power. For instance, when theconductive mount and the transparent window have dissimilar CTE,clamp-based mechanical fastening, and holding two conformal surface incontact can be implemented. Thermal pastes or flexible metals such asindium can be applied to increase thermal communication. Likewise,adhesives with flexibility can be implemented. When adhesives areimplemented they are preferably thin or otherwise thermally conductiveto allow appropriate heat transfer. For instance, thermally conductiveepoxies, films and tapes can be implemented.

The thermally conductive mount is attached to either the baseplate orother rigid mounting structure which can include passive or activecooling. The baseplate can be either mounted to another conductive heatsink for high power operation, or with sufficient surface area to allowenvironmental heat transfer. In one embodiment a water channel can beimplemented within the perimeter of conductive mount 76. Channels can bedesigned for laminar or turbulent flow. Alternatively, a thermoelectriccooler (TEC) can be implemented either in connection to the baseplate orin direct contact with the conductive mount. Addition of thermistorsprovide temperature feedback and allow controlled operation of activecooling techniques, by for example, the laser controller.

Heat spreader embodiment 152 shows one preferred orientation whereinpumping radiation enters gain medium 12 from the first surface.Alternatively, the orientation can be reversed. Further, conductivemount 152 can be placed such that conductive mount second surface 156Sis in thermal communication with optically transparent window firstsurface 152F. When oriented in such a manner thru-hole 158 can be sizedsmaller than the diameter of gain medium 12, still sufficiently large toallow the pump radiation to pass unobstructed.

FIG. 4B shows a heat spreader embodiment 150B. Heat spreader 150B issimilar to the heat spread as that shown in FIG. 4 with the addition ofthermally conductive mount 160. Thermally conductive mount 160 has afirst surface 160F, a second surface 160S, and a thru-hole 162. Secondsurface 160S is in thermal communication with optically transparentwindow 152F. Here, thru-hole 162 can be sized smaller than the diameterof gain medium 12, still sufficiently large to allow the pump radiationto enter through the thru-hole.

FIG. 4C shows a heat spreader embodiment 150C. Heat spreader 150C issimilar to that shown in FIG. 4B with the addition of an opticallytransparent window 166. Transparent window 166 has a first surface 166Fand a second surface 166S. The second surface of gain medium 12 inthermal communication with transparent window first surface 166F.Transparent window second surface 166S is in thermal communication withconductive mount first surface 156F. Here, thru-hole 158 can be sizedsmaller than the diameter of gain medium 12, sized sufficiently large toallow lasing radiation to pass unobstructed.

FIG. 4D shows yet another heat spreader embodiment. A heat spreader 150Dhas that shown in FIG. 4C with the addition of a thermally conductivemount 170. Thermally conductive mount 170 has a first surface 170F and asecond surface 170S. Here thermally conductive mount 170 is the samethickness or thinner than gain medium 12. Second surface 152S ofoptically transparent window 152 is in thermal communication withconductive mount first surface 170S. When thermally conductive mount 170is the same thickness as gain medium 12, then second surface 170S isalso in thermal communication with optically transparent window firstsurface 166F.

FIG. 4E shows a heat spreader embodiment 150E, wherein the heat spreaderis not in contact with the gain medium clear aperture. Here a conductivemount 182 has a open diameter 184, the diameter at least as large asgain medium diameter 12D. Gain medium 12 resides within heat spreader182, fitting within diameter 184. Gain medium 12 is in thermalcommunication with conductive mount 182 by physical contact withretainer lip 186. Retainer lip 186 has an inner diameter 188 at least aslarge as the gain medium clear aperture 12C. The gain medium can besecured by mechanically, for instance with a retaining ring or with anadhesive. The thermal communication can be increased with addition ofthermal past, thermal epoxy, solder, wettable metal such as an indium,or other such filler.

FIG. 4F shows a heat spreader embodiment 150F, another of the heatspreader embodiments where the heat spreader is not in contact with thegain medium clear aperture. Here, heat spreader 190 is a thermallyconductive material, deposited when viscous, then cured. Two nonlimitingexamples include thermally conductive epoxy and solder. In thisembodiment the gain medium resides with a V-groove 62V, the V-groovemachined within baseplate 62. The thermal epoxy is deposited and inthermal contact with the base of gain medium 12, not in contact withclear aperture 12C. If increased thermal management of the gain mediumis required heat spreader 190 can be deposited to encompass the entiregain medium, not deposited within the clear aperture.

FIG. 5A and FIG. 5B illustrates a bias method of operating the lasersystem of the present disclosure to reduce pulse to pulse jitter. Ingeneral, pulsed laser performance metrics include average power, peakpulse power, pulse energy, pulse duration, and pulse frequency. Theaverage power is the product of pulse energy and repetition rate. Peakpulse power is the product of the pulse duration and pulse energy, bothof which depend on the detailed pulse shape. Pulse frequency depends onthe pumping mechanism, modulation, and q-switch device employed. Forpassive q-switching, high frequency pulse rates can be increasing pumpintensity, but pulse-to-pulse jitter or timing jitter occurs due toinstability of q-switching. Instable q-switching arises from a varietyof factors which include residual population inversion in the gainmedium, recovery of the saturable absorber, thermal effects of theRE:XAB gain medium, and pump source fluctuation.

Timing jitter is inversely proportional to the slope the populationinversion density as it is pumped to a lasing state, or within a lasingwindow. Increasing the population density slope, decreasespulse-to-pulse jitter. One method of increasing the slope is throughcontrol of the pumping source and pump profile.

FIG. 5A is of a graph 250A, depicting a current pump profile 250A. FIG.5B is a block diagram 250B, depicting the bias method steps asillustrated in FIG. 5A. During operation the pump source receives pumpcurrent from the pump driver, the pump driver controlled by the lasercontroller. The pump current determines the amount of short wavelengthNIR optical radiation emitted by the pump source. The pump current has apump current profile 252. Pump current profile 252 is held at an initialcurrent L such that little or no optical radiation is not emitted fromthe pump source. A LD for example emits little to no optical radiationwhen operated at or below the laser threshold level. Current L can be atzero or any intermediate value about below the emission threshold of thepump source, thereby providing a pump source bias 254.

When a pulse is desired, the laser controller causes a current increase256 to a bias pump current I_(B). Bias pump current I_(B) causesincreased population density within the RE:XAB gain medium, but is belowa lasing threshold current IL, lasing threshold current sufficient tocause pulse emission if applied continuously. Bias pump current I_(B) isapplied for a bias duration TB. Bias duration TB is long enough to causeincreased population inversion density at about a predictable level.

A pump pulse 260 is delivered via a sudden increase in the current, pumppulse greater than a lasing threshold current IL. The pump pulse causesa sudden increase in population inversion density, resulting in a laserpulse emission 262. The sudden increase causes population inversiondensity slope to be high, shortening the temporal lasing window.

The pump current is reduced at point 266 after the emission of the laserpulse, down to after pulse current I_(AP). After pulse current I_(AP)can be at about any value between the zero and the bias current,depending on when the next desired pulse is desired and the residualpopulation inversion density. With the about predictable populationinversion level and steep population inversion slope, the lasing windowis shortened and pulse emission about temporally predictable loweringpulse to pulse jitter in continued operation.

The bias-method of operating the laser system can be implemented in avariety of ways. For instance, when continuous operation of the lasersystem is desired with identically temporally spaced laser pulses, thesame pump current profile can be implemented. Alternatively, a pluralityof pump profiles can be used to cause varying delays between pulses. Thepump profiles can be altered by the pump source bias applied, the biaspump current, or the pump pulse and durations of each. In one embodimentthe pump profile can comprise two pump sources with two differentwavelengths, for example 940 nm and 976 nm. Pump current profile 250A isshown as a step-function, in practice the pump current can be driven asshown, as a nonlinear continuous function, or as a series of pulses ofvarying amplitude and pulse width.

The pump profile can be constructed from one modulated source or aplurality of pump sources. For instance, one pump source can provide thebias pump current and the pump pulse by modulation of the current of theone pump source. Alternatively, a plurality of pump sources can beimplemented with one pump source providing the bias pump current and asecond pump source providing the pump pulse. Further a combination aplurality of bias pump current sources and a plurality of pump pulsesources can be provided.

The present embodiments and methods described in the present disclosureinvention have a variety of useful applications. In particular, thelasing emission bands in some embodiments are in about the so-called“eye-safe” region and can be utilized in applications in areas in whichintense laser pulses are normally not permitted. Applications includemachine vision in manufacturing processes, robotics machine visionincluding autonomous car guidance, range finding, LIDAR/LADARapplications, biomedical application, target designation, and other suchapplications.

From the description of the present disclosure provided herein oneskilled in the art can manufacture apparatus and practice the methodsdisclosed in accordance with the present invention. While the presentinvention has been described in terms of particular embodiments andexamples, others can be implemented without departing from the scope ofthe present invention. In summary, the present disclosure abovedescribes particular embodiments. The invention, however, is not limitedto the embodiments described and depicted herein. Rather, the inventionis limited only by the claims appended hereto.

What is claimed is:
 1. A method of operating a q-switch RE,XAB laser,the steps comprising: providing a pump bias current to a pump source,the pump source directed to an RE:XAB gain medium, the RE:XAB gainmedium within a resonator cavity, where X is selected from Ca, Lu, Yb,Nd, Sm, Eu, Gd, Ga, Tb, Dy, Ho, Er, and where RE is selected from Lu, Y,Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Pr, Tm, Cr, Ho, with a bias currentlevel below a lasing threshold of the RE:XAB gain medium; providing apump pulse to the gain medium, the pump pulse of the lasing threshold ofthe RE:XAB gain medium, the pump pulse causing the RE:XAB gain medium toemit a laser pulse; and reducing the pump bias current to at least belowthe gain medium lasing threshold, the combination of the pump bias, thepump pulse, and the pump reduction having a current profile.
 2. Themethod of claim 1, wherein the pump source has optical output peakedbetween about 750 nm to 1150 nm.
 3. The method of claim 1, wherein thelaser system has a heat spreader.
 4. The method of claim 1, furthercomprises a polarizer.
 5. The method of claim 4, wherein the polarizeris within the laser resonator.
 6. The method of claim 1, wherein thepump bias current is modified based on characterized thermal performanceand thermal variation.
 7. The method of claim 1, wherein the pump sourcehas optical output peaked between about 1450 nm to 1600 nm.
 8. Themethod of claim 1, wherein the pump source has optical output peakedbetween about 1800 nm to 2100 nm.
 9. The method of claim 1, wherein thepumping source resonantly pumps the RE:XAB gain medium with opticaloutput having a peak wavelength within 200 nm of the laser system'semission wavelength.
 10. The method of claim 1, wherein the laser systemfurther comprises a photodetector, the photodetector detecting pulseemissions.
 11. The method of claim 10, wherein the pump bias currentprofile is modified based on temporal feedback from the photodiode. 12.The method system of claim 10, wherein the photodiode is an avalanchephotodiode.
 13. The method of claim 1, wherein the current profilereduces pulse-to-pulse jitter.
 14. The method of claim 1, wherein thepump pulse is temporally shorter than the excited state lifetimes of theRE:XAB gain medium.
 15. The method of claim 1, wherein the currentprofile is operated for asynchronous pulse frequency operation.
 16. Themethod of claim 1 wherein the current profile is a superposition of aplurality of pump sources.
 17. The method of claim 1, wherein the RE:XABgain medium is double-end pumped.
 18. The method of claim 17, wherein atleast one of the plurality of pump sources have different emissionwavelengths than another of the plurality of pump sources.
 19. Themethod of claim 17, wherein the pump pulse is a plurality of pulses. 20.The method of claim 3, wherein the laser system has an opticallytransparent heat spreader.
 21. The methods 20, wherein the opticallytransparent heat spreader is sapphire or synthetic diamond.